U.S. Pat. No. 3,432,771, Hardy et al., issued Mar. 11, 1969 discloses an apparatus for changing the direction of a light beam in an optical cavity. The cavity consists of a focussing objective, located between two reflectors, such as curved mirrors. The relative position of one center of curvature with the other center of curvature can be controlled by positioning of one of the mirrors. Points on the reflectors are located at the object and the image positions for the objective. When the active medium is suitably excited, the orientation of the lasing mode, and hence the position of the spots of light, is determined by the effective angular positioning of the reflectors.
U.S. Pat. No. 3,480,875, Pole, issued Nov. 25, 1969 disclosed a laser cavity which was set up between a pair of plane mirrors. At least one active laser element is located between the mirrors. A pair of lens systems are positioned between the mirrors so that they have a common focal plane between them. A Kerr cell, polarizes, and a compensator suppress light oscillation along certain reflector paths within the cavity, thereby setting up preferred modes of oscillation along other paths. Laser emission occurs along the preferred paths.
U.S. Pat. No. 3,597,695, James E. Swain, issued Aug. 3, 1971 disclosed an apparatus for amplifying laser light by multiple passes through a lasing material in a single laser cavity. A single amplifier stage achieved what has been accomplished by several stages. This is accomplished by a switching mechanic which directs a laser beam into and out of the cavity at selected time intervals, thereby enabling amplification of low intensity laser pulses to energy level near the damage limits of the optical components of the system.
U.S. Pat. No. 4,191,928, John L. Emmett, issued Mar. 4, 1980 disclosed a high energy laser system using a regenerative amplifier, which relaxes all constraints on laser components other than the intrinsic damage level of matter, so as to enable use of available laser system components. This can be accomplished by use of segmented components, spatial filter.
"A survey of laser beam deflection techniques," by Fowler and Schlafer, Proceedings of IEEE, vol 54, no. 10, pages 1437-1444, 1966.
The control of laser beam positioning has become a key element in many field of applications such as image processing, graphic display, materials processing, and surgical applications involving precision tissue removal.
Many techniques have been developed for the controlling of the laser beam direction. For the purpose of this invention, this discussion will be limited to the speed, accuracy, and the scan angle range of different devices used in a random access mode.
Galvanometer mirror scanners have a large scan angle range. However, the mechanical response due to the balance of the coil and the applied magnetic field is limited to a few hundred Hertz. The settling time and the oscillation about the equilibrium point further limits the accuracy attainable with such devices.
Mirrors positionable with piezo actuators are capable of accurate hunt free movement response of up to tens of kilo-hertz, depending on the design of the mounts. The typical scan angle is in the order of few milli-radiant. Methods to enhance the scan angle has been proposes by J. Schlafer and V. J. Fowler, "A Precision, High Speed, Optical Beam Scanner," Proceedings, International Electron Devices Meeting, 1965. In their report, multiple piezo-mirrors were used to intercept the laser beam, such that the scan angle of the each scanner is contributing to the total of the effect, which is the sum of all scan angles. This device requires many individual scanner units, which multiplies in economic cost with the number of units, and the mirror size also limits the number of units to be used before the beam will miss the last mirror.
Furthermore, both of the above methods are applicable in one dimensional scanning only. For two dimensional scans, an additional unit, which is either and an identical or a mix with another device must be provided for the scanning in the other dimension, doubling the cost, and space requirement.
In U.S. Pat. No. 3,480,875, R. V. Pole, has disclosed a scanning laser device, in which the spatial orientation of the laser beam in the resonant cavity is controlled by passing through a combination of a retardation plate and a Kerr cell inside the laser cavity. At a specific angle as determined by the Kerr cell, loss is minimum for the laser beam, and therefore the laser beam will oscillate in that preferred direction. While this method allow a scanning of large angles, the scan speed is limited by the laser build-up time, for which the laser beam intensity will be re-established at each new beam direction. Another drawback of this arrangement is the variation in the laser intensity, during the laser build-up.
In U.S. Pat. No. 3,432,771, W. A. Hardy, disclosed another scanning laser, in which the optical cavity consists of a focusing objective, and spherical reflectors or equivalent optics which consists of a lens and a plane mirror. The scan angle is magnified most effectively in an optical arrangement that the two end reflectors forms a nearly concentric cavity with the focusing lens at the center of focus. The drawback is the cavity is tolerant of diverging beam to build up inside the cavity as illustrated in the FIG. 1 of the patent, hence that laser output was high content of multiple transverse mode. With an increase of the radius of curvature of the scan mirror and keeping its location fixed, the multi-mode content can be reduced, but the scan range with approach to that of the actual scan angle with a possibly a small magnification factor. As suggested by its preferred embodiment with an electro-optical beam deflector, the scan angle will be only a few milli-radiant if a near diffraction-limited laser beam is to be produced.
There has not been a successful apparatus and method to overcome the above deficiencies of the prior art until the emergence of the present invention.